Safety isolation is an important consideration in the design of any electrical product that can expose its users to potentially hazardous voltage levels. Manufacturers of such products need to ensure that their products are designed to adequately safe-guard against electrical shock.
In any safety isolation scheme, there is a certain amount of insulation required to create a protective isolation barrier between primary and secondary circuits. A primary circuit is a circuit that is directly connected to line voltage and therefore clearly has the potential to reach hazardous voltage levels. A secondary circuit is not electrically connected to the primary circuit (that is, there is no conductive connection between the secondary circuit and primary circuit) but could experience hazardous voltage levels if insulation fails.
Underwriters Laboratories Inc. (UL) has developed a safety standard called UL 60950-1 (second edition), which specifies a high level of safety isolation to protect against risk of injury due to electric shock. The UL 60950-1 standard is based on the International Electrotechnical Commission (IEC) standard, IEC 60950-1 (second edition). The UL 60950-1 standard defines five categories of insulation: functional insulation; basic insulation; supplemental insulation; double insulation; and reinforced insulation. Functional insulation does not protect against electrical shock. Basic insulation is a single level of insulation that provides basic protection against electric shock. Supplemental insulation is independent insulation that can be applied in addition to basic insulation to reduce the risk of electrical shock in the event of a failure of the basic insulation. Double insulation comprises both basic insulation and supplemental insulation. Thus, basic insulation provides a single layer of insulating barrier between primary and secondary circuits, whereas double insulation provides two layers of insulating barrier between primary and secondary circuits. Reinforced insulation is a single insulation system that provides electrical shock protection equivalent to double insulation.
If a secondary circuit is not user-accessible, a basic level of isolation protection, that is, basic insulation, may be acceptable. Safety needs for products that include user-accessible secondary circuits, on the other hand, demand two levels of protection provided by double insulation or its equivalent in reinforced insulation.
Although the UL 60950-1 standard is intended for use with information technology equipment, it has been adopted for many other products and is considered the premier standard for product designs that require galvanic isolation. One product that requires galvanic isolation is the current sensor. Some current sensors use a magnetic field transducer (for example, a Hall effect or magnetoresistive transducer) in proximity to a current conductor. The magnetic field transducer generates an output signal having a magnitude proportional to the magnetic field induced by a current that flows through the current conductor.
The magnetic field transducer is sometimes provided as an integrated circuit (IC) in an IC package that also contains a current conductor. Illustrative current sensors of this type are sold under part numbers ACS712 and ACS758xCB by Allegro MicroSystems, Inc. of Worcester, Mass. 01615, the Assignee of the subject application.
Various parameters characterize the performance of such current sensors, including sensitivity. Sensitivity is related to the magnitude of a change in output voltage from the magnetic field transducer in response to a sensed current. The sensitivity of a current sensor is related to a variety of factors. One important factor is the physical separation between the magnetic field transducer and the current conductor. Integration of the current conductor into an IC package allows for close and precise positioning of the current conductor relative to the magnetic field transducer.
At present, to meet the more stringent requirements for double or reinforced insulation, applications that use such integrated circuit current sensors rely on basic insulation provided by the current sensor's package assembly, e.g., molding plastic or other insulation, in conjunction with an external, supplemental level of insulation provided by the use of a discrete opto-isolator or similar device.